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A Full Moon Feature

Step outside and see the blue moon New Year's Eve.
A bright full moon.

The night sky this New Year's Eve will feature a "blue moon." You may need to move away from street lights and city lights to enjoy the full impact of this event, but it will be worth the effort. It's been twenty years since there's been a blue moon on New Year's Eve, and it will be another twenty years before it happens again. So what exactly is it?

The usual definition of a blue moon is the second full moon in a calendar month. December 2009 qualifies because there was also a full moon on Dec. 2.

However, some references consider this an incorrect description caused by a mistake in a Sky and Telescope magazine article published more than sixty years ago. Instead, they define a blue moon as the third full moon in a season that features four full moons.

Now, if you're as intrigued (or confused) as I am, do a bit of Web research on your own. The definitions are listed on this site. An interesting Sky and Telescope article detailing the history of the phrase "once in a blue moon," appears here.

If you figure it out, please register or sign in and add a comment to share with our readers.

And be sure to enjoy the New Year's Eve show. Happy 2010!

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Full moon is a lunar phase

Submitted by sherry william on Sat, 03/27/2010 - 1:19am.

Full moon is a lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent ecliptic longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun. As seen from Earth, the hemisphere of the Moon that is facing the earth the near side is almost fully illuminated by the Sun and appears round. Only during a full moon is the opposite hemisphere of the Moon, which is not visible from Earth the far side, completely unilluminated. The time interval between similar lunar phases the synodic month is on average about 29.53 days. Therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because lunar months have a whole number of days, lunar months may be either 29 or 30 days long.